One common form of viscometer comprises two coaxial cylinders (cylinder-in-cylinder) which are rotated relative to one another while measuring, either visually or electronically, the torque, or torque equivalent, required to achieve a differential rotation speed. The flow characteristics of the fluid can be determined by interposing the fluid in an annular gap between the cylinders and for a known differential rotational speed, measuring the torque, or torque equivalent. By factoring in the physical dimensions and the drag associated with bearings or seals of the viscometer that can affect torque measurement, the viscosity of the fluid can be calculated for a particular shear rate. Typically, a viscometer is driven at a single speed and the viscosity calculated at a single shear rate to allow relative comparison of fluids. However, if desired the viscometer can also be used to more fully characterize a fluid, by measuring torque over a range of differential rotational speeds.
In certain applications, viscometers are used to continuously monitor a fluid used in or produced by a device or process. The fluids can be either totally liquid or a liquid containing particulate. One method for using known coaxial cylinder viscometers in these applications is to put the viscometers in line with the fluid flow. Problems with this method include the complexity of designing the viscometers into the flow circuit, the difficulty in replacing components of the viscometers should failure occur, and accuracy issues should the fluid flow past the viscometers vary from a constant rate.
One way of overcoming some of the problems associated with mounting cylinder-in-cylinder viscometers in line with the flow path is to mount the viscometers outside the main flow path. In this arrangement, the outer cylinder of the viscometers is capped to form a cup-like structure with the inner cylinder or bob inside the cup. This allows the drive for the differential rotation to be mounted quite close to the rotating elements for a more compact design and also allows maintenance issues to be more easily addressed.
A problem with prior bob-in-cup viscometers used to continuously monitor a fluid used in or produced by a device or process is that a pump or other hardware is needed to control the fluid flow through the viscometers, which adds to the cost and complexity to using the viscometers. Another problem with prior bob-in-cup viscometers is that, when used to accurately measure low viscosity fluids containing particulate, particulate settling can occur resulting in inaccurate viscosity calculation. Thus, careful placement of prior bob-in-cup viscometers is critical to proper operation. Also, such viscometers are potentially subject to a number of possible sources of error due to unwanted friction and/or drag effects.